1. Field of the Invention
The present invention relates to chip type filters and, more particularly, to a chip type filter in which resonators having a quarter wavelength, for example, are electro-magnetically coupled to one another.
2. Description of the Related Art
FIG. 15 is a perspective view showing an example of a conventional chip type filter. The chip type filter 1 includes a dielectric substrate 2. A ground electrode 3 is formed on the entire surface of one side of the dielectric substrate 2. Two linear pattern electrodes 4a and 4b are formed on the other side of the dielectric substrate 2 so that they are opposite to the ground electrode 3. One end of each of the pattern electrodes 4a and 4b is connected to the ground electrode 3 through an end face of the dielectric substrate 2. Input/output electrodes 5a and 5b are formed so that they extend respectively from the middle of the pattern electrodes 4a and 4b to opposite sides of the dielectric substrate 2. These input/output electrodes 5a and 5b are formed at intervals from the ends of the pattern electrodes 4a and 4b, respectively, which are connected to the ground electrode 3. A micro strip line is formed by the dielectric substrate 2, ground electrode 3, and pattern electrodes 4a and 4b. Each of the pattern electrodes 4a and 4b forms a resonator having a quarter wavelength, and a filter is formed as a result of the electromagnetic coupling of those resonators. The input/output impedance of this chip type filter 1 is adjusted by adjusting the intervals between the input/output electrodes 5a and 5b and the ground electrode 3.
The length L of a pattern electrode of a strip line resonator is generally expressed by L=.lambda.(4(.epsilon..sub.re)1/2) where .lambda. represents the wavelength and .epsilon..sub.re, represents the effective dielectric constant of the dielectric substrate. As apparent from the equation, the size of a chip type filter may be reduced by using a dielectric material having a high dielectric constant. However, since the use of a dielectric material having a high relative dielectric constant results in problems associated with the temperature characteristics thereof, the relative dielectric constant of the materials actually used is up to about .epsilon..sub.r =100.
If this relative dielectric constant is treated as the effective dielectric constant as it is, the pattern electrode will be very long, e.g., as long as 7.5 mm when the frequency used is 1 GHz. When linear pattern electrodes are used, since the length along which they face each other is long, there will be excessive coupling between the electromagnetic fields of those resonators. In order to optimize the electromagnetic coupling, a large interval must be provided between the two pattern electrodes, which results in an increase in the size of the chip type filter.
In order to avoid the above-described problem, the pattern electrodes 4a and 4b may be provided in the form of a spiral as shown in FIG. 16. By providing the pattern electrodes 4a and 4b in the form of a spiral, this chip type filter 1 can be made smaller than a chip type filter having linear pattern electrodes. Further, the length in which the pattern electrodes 4a and 4b face each other is small, which results in weaker electromagnetic coupling. Thus, it is possible to make the pattern electrodes 4a and 4b closer to each other to provide a smaller chip type filter without making the coupling of electromagnetic fields too strong.
In a chip type filter having spiral-shaped pattern electrodes as described above, the interval between one end of a pattern electrode and an input/output electrode becomes small as a result of the reduction of the size of the filter. This results in problems such as increased insertion loss due to difficulty in matching input and output impedance. The width of the pattern electrodes may be reduced to correct the mismatch between input and output impedance. However, this method results in deterioration of Q.
It is therefore an object of the present invention to provide a chip type filter which can be fabricated in a small size, allows the matching of input and output impedance, and provides preferable characteristics.